Carol Bradford: Japanese beetles likely damaging hibiscus

CAROL'S TIP: Italian arum (A. italicum) is out of sync with the rest of the garden. The new leaves are coming up now. Plants that are summer dormant include hardy cyclamen, Oriental poppies and grape hyacinths. Avoid damaging their foliage when cutting other plants back for the winter.

Courtesy Carol Bradford

Dear Carol: We have had three hibiscus bushes for over five years. The flowers are incredible -- abundant and huge; absolutely beautiful! However, this year for the first time, the leaves were eaten by something until they look like lace.

I did not see any Japanese beetles or slugs and have never had anything like this before, nor did it affect the blooms. Nothing else in the garden seemed affected. Can you tell me what it might be and if there is any remedy for next year? -- J.P., Manlius.

Dear J.P.: There are only a few pests that chew on the foliage of hardy hibiscus. I have seen the lacy-looking leaves you describe, and I attributed the damage to Japanese beetles. Hibiscus are in the mallow family, and mallows are right up on the list of Japanese beetle favorites, even higher ranked than roses, raspberries and grapevines.

Once you recognize Japanese beetles, they are hard to miss if you are on the lookout for them. They are oval, about half an inch long, and shiny, metallic, coppery-green colored.

A few beetles can do much damage, especially to a plant like hardy hibiscus which is slow to emerge in the spring and probably still has undeveloped leaves at the end of June when the beetles start feeding. They feed on the upper surfaces of leaves, primarily from midmorning to late afternoon.

The female beetles laid their eggs in the soil and the little grubs are chewing on turfgrass roots now. Soon they will stop feeding and move deeper into the ground, where they spend the winter. In the spring, as the soil warms, the grubs move up, feed on the grass roots for a little while, then pupate and emerge as adult beetles around the end of June.

As this is the first time you've had problems on the hibiscus and you have been growing them for several years, I would not take any preemptive action. Next season, keep an eye on the hibiscus as they grow. Start scouting the last week in June. If you catch the beetles in action, handpicking works to control them. Insecticides are also available to control the adults.

Traps do not prevent or even reduce the damage to plants and do not lower the numbers of grubs in the soil either, according to research. The traps can be used to monitor the appearance of the beetles in early summer and to track new infestations.

It's probably not a good idea to plant varieties that are notably susceptible to Japanese beetle damage near large areas of untreated turf. The beetles lay their eggs in nearby soil and although the adults do fly, there's no reason to make food easily available to them.

Treating the soil for grubs in the spring is a waste of time and money. Treat in late summer, if at all.

Carol T. Bradford, of Syracuse, has been gardening in Central New York for more than 25 years. Send questions in care of Home & Garden, The Post-Standard, P.O. Box 4915, Syracuse, NY 13221, by fax to 470-2111 or by email to features@syracuse.com. Letters might be edited for space and clarity. »